Hydraulic windmills involve the rotation of the windmill blades to effect pumping of a hydraulic fluid through a closed loop which incorporates either a fluid driven generator or a friction heater for transforming or converting the mechanical energy to electric or thermal energy, or both as desired. My U.S. Pat. No. 3,952,723 covers such a basic hydraulic windmill wherein the power is transmitted through the closed loop hydraulic circuit, and wherein the maximum turning speed of the rotor via the windmill blades is governed by a pressure compensating flow control valve. In that patent, the rotation of two or more windmill blades about a generally horizontal axis and through a rotatable shaft is converted to a reciprocating pumping action by means of a suitable crank coupled to the shaft. The reciprocation of a piston within a closed cylinder functions to periodically force hydraulic fluid in a closed loop system to enter the cylinder during the suction stroke of the piston and to be forced from the cylinder during the expansion stroke. The hydraulic liquid may move through a fluid rectifier and is forced to pass through a hydraulic motor for converting the fluid flow into electrical energy by means of an electrical generator coupled to the hydraulic motor. Alternatively, by forcing the hydraulic fluid to pass through a restrictor, by means of a heat exchanger downstream of the restrictor, the thermal energy generated in passing through the restrictor may be transmitted via the further working fluid external of the hydraulic circuit.
It has been determined that with relatively large scale hydraulic windmills, the hydraulic circuit may also be used to govern an overpower condition resulting from excessive wind velocity or at times of loss of hydraulic fluid in the closed loop hydraulic circuit.